The present invention relates to a method of producing plastic articles which constitute tubes or include tubular portions, and more particularly to improvements in a method of producing plastic articles which exhibit one or more undercuts in the form of internal grooves or recesses provided in a tubular portion which is flanked by tubular portions having smaller inner diameters. Such undercuts are often necessary in fittings in order to receive sealing elements in the form of gaskets or packings. Still more particularly, the invention relates to improvements in the molds of injection molding or analogous machines, and in a method which can be practiced by resorting to such molds.
Flanged fittings or pipe connectors (e.g., elbows, return bends, tees, crosses, reducing couplings or straight unions) are often provided with undercuts to accommodate sealing elements for the ends of flexible or rigid pipes which are inserted into the fittings. In accordance with a presently prevailing technique, undercuts in the flanges of fittings which are produced in injection molding machines are formed by resorting to one or more secondary operations which contribute to the cost of such articles. For example, it is known to provide a flange of a fitting with a relatively thick circumferentially complete rib or bead which is thereupon treated from within (e.g., by resorting to milling or turning tools) in order to form therein a ring-shaped internal recess or groove for reception of a sealing element. The formation of such internal groove takes place subsequent to ejection of the article from an injection molding machine so that the interval which is required for completion of an undercut fitting is much longer than the interval which is required for completion of the injection molding cycle.
It was further proposed to make undercut fittings in specially designed injection molding machines wherein the injection molding cycle precedes a blowing cycle serving for the formation of an undercut in one or more flanges of the fitting. It is also known to produce fittings in conventional injection molding machines which furnish fittings with flanges having internal surfaces of constant diameter, and to thereupon introduce such fittings into a blow molding machine which expands a selected portion of a flange so as to provide the internal surface of the flange with a groove for reception of sealing means. Such procedures are not satisfactory because the wall thickness of a blow molded undercut portion invariably deviates from the wall thickness of neighboring portions of the finished article.
In accordance with still another presently known technique, that flange of a fitting which is to be provided with an undercut resembles an outwardly flaring hollow frustum of a cone, and the outermost portion of such flange is thereupon treated to reduce its dimensions, i.e., the inner diameter of the thus contracted portion is less than the inner diameter of the neighboring portion. The neighboring portion can receive a sealing element which is held against axial movement by the contracted portion of the flange. The just described procedure shares the drawbacks of previously outlined techniques, i.e., the internal groove of the undercut portion must be formed in a separate operation which follows the making of the fitting. Moreover if a fitting, whose flange has been formed with an undercut in accordance with the last mentioned prior procedure, is used in pipe lines for hot fluids, the connection between the flange and a flexible or rigid conduit rapidly develops a leak. This is attributable to a peculiarity of thermoplastic synthetic plastic materials which is known as thermal restoration. Briefly stated, the molecules of thermoplastic material assume a predetermined orientation during introduction of plasticized material into a mold. Thus, the orientation of molecules in a flange which, at the time of introduction of plasticized material into the molding cavity, resembles a hollow conical frustum is different from the orientation of molecules in a cylindrical flange. When the flange is heated subsequent to formation of an undercut therein (by causing the free end of the flange to shrink), the molecules tend to assume the original orientation which is characteristic of a hollow frustoconical flange with the result that the flange opens up and permits communication between the confined fluid and the surrounding atmosphere.